1. Field of the Invention
The present invention relates to a time-varying image signal coding/decoding system, and more particularly to a time-varying image signal (moving picture signal or video signal) coding/decoding system using an interframe differential orthogonal transform coding method combining the prediction of interframe differentials and orthogonal transform.
2. Description of the Prior Art
In a time-varying image signal coding/decoding system of this kind according to the prior art, the frame memory in the time-varying image signal decoding unit is periodically refreshed in order to prevent any error on the transmission path from deteriorating the picture quality on the receiving side. Where an interframe differential orthogonal transform coding method or a orthogonal transform interframe coding method is used, since the content of the frame memory is a transform coefficient after orthogonal transform, time-varying image signals entered into the time-varying image signal coding unit are subjected to orthogonal transform, and quantized signals are transmitted at a rate of, for instance, one block line per frame as refreshing signals to refresh a whole frame in the frame memory in the time-varying image decoding unit at fixed intervals. Incidentally, where coding takes place in units of a two-dimensional block of I lines by J pixels each, for example, I lines of signals are collectively referred to as one block line.
In the conventional time-varying image signal coding/decoding system mentioned above, all the transform coefficient components of the signals resulting from the orthogonal transform coding of time-varying image signals entered into the time-varying image signal coding unit are collectively used as refreshing signals. However, picture quality deterioration on the receiving side due to any error on the transmission path usually results from a partial error in the transform coefficient. The time-varying image signal coding/decoding system according to the prior art, which collectively refreshes even the components needing no refreshing, involves the problem of wastefulness. Furthermore the conventional time-varying image signal coding/decoding system often accomplishes refreshing in units of block lines, and in that case there is the additional problem of restricting the efficiency of coding because information for the refreshing purposes accounts for a high proportion in each frame.
The above-mentioned time-varying image signal coding/decoding system according to the prior art is described in detail in a paper entitled "International Standardization of Video Codes for ISDN Videoconferencing and Videophone Services" in NTT Review, Vol. 2, No. 3, May 1990 (p.p. 110-117).